1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus such as a laser printer using the electrophotographic process, comprising comparing means for comparing a threshold signal with image density data (referred to as density data hereinafter) corresponding to an image to be formed.
2. Description of the Related Art
Conventionally, when a half tone image is printed on a piece of paper by a printer, density data of respective pixels are sent from an image reader to a printer controller, wherein each density data represents a gradation of each pixel. The printer controller generates a printing signal for controlling the printer to print each dot utilizing the dither method according to the received density data and outputs the printing signal to the printer. In the printer of this type, the sizes of respective dots are set at substantially the same.
Recently, the pulse modulation method has been used for half tone representation. In this method, the density of one dot is represented by a gradation signal of, for example, 8 bits, and the pulse width of the printing signal is modulated according to the density data so as to change a dimension of each dot to be printed in order to reproduce a half tone image having 256 gradation steps. In this case, the half tone image can be printed without deterioration of the resolution of the printed image. For example, in the laser printer, a semiconductor laser diode emits a beam of light for a period of the pulse width so as to form each dot having a size corresponding to the pulse width.
In order to generate the printing signal using the pulse width modulation method, for example, there has been used a method of comparing the density data with a predetermined pattern signal having a constant period and a constant amplitude such as a sawtooth wave signal. In a method using the sawtooth wave signal as a threshold signal, the sawtooth wave signal having a constant amplitude and a constant period is inputted to an inverted input terminal of a comparator. The period of the sawtooth wave signal is set so as to correspond to the output period of the density data. A gradation signal of, for example, 8 bits which has been received from the host computer such as an image reader is converted into an analogue voltage for one period of the sawtooth wave signal, and the converted analogue voltage is inputted as density data to a non-inverted input terminal of the comparator. In this case, a pulse signal for printing a dot is generated for a period when the density data are larger than the level of the sawtooth wave signal for one period thereof. This pulse signal is outputted to the printer for a period which is in proportion to the density data, and then, a dimension of each dot is modulated according to the pulse width of the pulse signal.
Further, in the triangular wave method which is a modification of the aforementioned sawtooth wave method, a triangular wave signal having a constant period and a constant amplitude is used instead of the sawtooth wave signal in order to improve the gradation characteristic. In this case, one pulse signal is generated for a period of two dots.
The composition of the printer controller using the aforementioned sawtooth wave signal is simple. However, for example, when a pulse signal is generated with a period of non-integer times one dot, there is such a problem that it is necessary to alter the output period of the sawtooth wave signal.